Data from: Asymmetrical nature of the Trollius-Chiastocheta interaction: insights into the evolution of nursery pollination systems

The mutualistic versus antagonistic nature of an interaction is defined by costs and benefits of each partner, which may vary depending on the environment. Contrasting with this dynamic view, several pollination interactions are considered as strictly obligate and mutualistic. Here, we focus on the interaction between Trollius europaeus and Chiastocheta flies, considered as a specialized and obligate nursery pollination system – the flies are thought to be exclusive pollinators of the plant and their larvae develop only in T. europaeus fruits. In this system, features such as the globelike flower shape are claimed to have evolved in a coevolutionary context. We examine the specificity of this pollination system and measure traits related to offspring fitness in isolated T. europaeus populations, in some of which Chiastocheta flies have gone extinct. We hypothesize that if this interaction is specific and obligate, the plant should experience dramatic drop in its relative fitness in the absence of Chiastocheta. Contrasting with this hypothesis, T. europaeus populations without flies demonstrate a similar relative fitness to those with the flies present, contradicting the putative obligatory nature of this pollination system. It also agrees with our observation that many other insects also visit and carry pollen among T. europaeus flowers. We propose that the interaction could have evolved through maximization of by-product benefits of the Chiastocheta visits, through the male flower function, and selection on floral traits by the most effective pollinator. We argue this mechanism is also central in the evolution of other nursery pollination systems.Seed set in the studied Trollius europaeus populationsData on the number of produced seeds from Trollius europaeus populations. Number of ovules (column ovules_sum) and developing seeds (column seed_sum) were counted in five carpels per fruit.DRYAD_seeds_lowlands09-11.xlsxSeed mass and germination in the four Trollius europaeus populationsMass measured on 25 seeds per fruit (column mass; in mg). Germination is measured as the number of germinating seeds form 25 initial ones, after two weeks (column germination). Seeds used for germination tests were the same as for weight measurement. For details on the germination trial please refer to the publication.DRYAD_germination_lowlands11.xls